Particle Size Distribution in a Transparent Cr Diesel Engine Fuelled with Rme and Gtl by Means of Multiwavelength Spectroscopy

In this paper, broadband UV–NIR flame emission measurements were performed in an optical Common Rail (CR) diesel engine in order to evaluate the formation process of soot particles with high spatial and temporal resolution. The measurements were carried out in a direct injection (DI) CR transparent research diesel engine equipped of a Euro 5 multi cylinder head. Two alternative fuels, chosen as representative of the first generation biofuel, rapeseed methyl ester (RME), and of the second generation biofuel, gas to liquid (GTL), and a commercial diesel were used. A comparison between the soot (particle) size distribution measured into the cylinder by means of a numerical procedure applied to optical data and those at the exhaust by means of an electrical low pressure impactor (ELPI) was performed. The engine condition 1500 rpm x 2 of break mean effective pressure (BMEP) was analyzed because it is characteristic of the New European Driving Cycle (NEDC). Introduction The internal combustion engines (ICEs) always generates some undesirable products which are finally emitted at the exhaust. Several factors can contribute to the pollutant emissions: vehicle fuel systems give off unburnt fuel vapors, and open-vented engine crankcases give off escaped combustion products and vaporized lubricating oil. Nevertheless, the exhaust emissions are mainly influenced by the fuel combustion process. A perfect combustion of any kind of fuel would involve complete oxidation of the entire sample with maximum heat production and no pollutant emissions. This would require complete mixing of exactly reacting quantities of the pure fuel and O2 with the addition of an appropriate amount of heat. For hydrocarbon fuels, the only products of combustion would be CO2, water vapor and heat. Despite the improvement of the combustion and engine hardware a perfect combustion does not yet exist and particulate matter, NOx and other pollutant are emitted. Concerns about the increasing crude oil price, the accelerated global warming and the particle emissions, have led to growing worldwide interests in renewable energy sources such as biofuels for their potential to reduce both CO2 emissions and particulate XXXV Meeting of the Italian Section of the Combustion Institute 2 matter mass emissions. Biofuels are produced from renewable biological sources. They are generally divided into primary and secondary generation fuels. First generation biodiesel is produced from vegetable oils and animal fats through a transesterification process. The 2nd generation of alternative fuel was obtained from the well-known Fischer-Tropsch synthesis process. The combustion process and then the particle emissions are strongly influenced by the physical properties and chemical composition of the biofuel [1, 2]. In particular, a larger amount of particles smaller than 100 nm (ultrafine) are emitted [1, 2]. Their contribution on the mass is negligible, for this reason a particle number regulation has been introduced from Euro5b [3]. In order to reduce the particle emission it is necessary to understand the complex phenomena occurring in the cylinder engine in terms of their formation and oxidation. In this paper, broadband flame emission measurements from UV to near IR were performed in an optical Common Rail (CR) diesel engine in order to evaluate the effect of biofuels on the formation process of soot particles. The measurements were carried out with high spatial and temporal resolution in the combustion chamber. Two alternative fuels: rapeseed methyl ester (RME) and gas to liquid (GTL), and a commercial diesel fuel were used. The in-cylinder measurements were compared with those at the exhaust by means of an electrical low pressure impactor (ELPI). Engine, Engine Operating Conditions and Fuels The optical single-cylinder research engine used for combustion diagnostics was equipped with the combustion system architecture and injection system of a fourcylinder standard Euro5 engine. The engine had bore of 85 mm, stroke of 92 mm and compression ratio of 16.5:1 and operated in continuous mode. It is also equipped with common rail injection system and a solenoid driven injector (7 hole nozzle, 440cm/30s). The exhaust gas recirculation (EGR) and variable swirl actuator (VSA) were managed by external devices. The amount of EGR and of VSA was set at 57% and 66%, respectively. More details and specifications are reported in the reference [4]. All the measurements were performed using a commercial European low sulfur diesel fuel (REF), a rapeseed methyl-ester (RME), representative of the most widespread FAME fuel, and GTL, representative of a FT fuels. The main properties of the fuels are reported in ref [4]. GTL had lower density and viscosity and higher cetane number (CN) and low heating value (LHV) than reference diesel. On the other hand RME had higher density and viscosity, and lower CN and LHV than REF. Moreover, RME had high percentage of oxygen in its formula (10.5%, m/m). The engine operating condition investigated was representative of the engine behavior on new European driving cycle (NEDC) when installed on a D-class vehicle and was widely investigated in previous work [4]. It corresponds to 1500 rpm x 2 bar of brake mean effective pressure (BMEP). The injection parameters were set in the electronic control unit (ECU) that manages the Common Rail XXXV Meeting of the Italian Section of the Combustion Institute 3 injection system. The injection strategies consisted of two injections per cycle, pilot and main, with an injection pressure of 500bar at 1500 rpm. For all the fuels investigated the start of pilot and main injections were kept constant. The pilot energizing time also was fixed; while the main energizing times of RME were longer with respect to the others fuels. Optical Experimental Apparatus and Theory The engine layout with the experimental apparatus is widely discussed in reference [4, 5].The flame emissions were acquired through the piston crown window and 45° mirror, placed in the elongated piston. The broadband UV-near infrared (NIR) flame emissions were collected and focused on the entrance slit of a spectrograph through an UV objective (Nikon 78 mm f/3.8). Spectrograph had 15 cm focal length, f/4 luminous, and was equipped with a grating of 300 grooves/mm, with a dispersion of 3.1 nm/mm. An entrance slit width of 100 μm was used. The spectral image formed on the spectrograph exit plane was matched with a gated intensified CCD (ICCD) camera (512 x 512 pixels) with 24x24 μm pixels. The ICCD had high sensitivity in the UV-Visible range. Data were detected with the spectrograph placed at a central working wavelength of 350 nm and with the intensifier-gate duration of 55 in order to have a good accuracy in the timing of the combustion onset. In particular, this time corresponds to 0.5 crank angle degrees at 1500 rpm. Moreover, in order to detect soot temperature and concentration by means of the two-color pyrometry method [6] digital imaging analysis was performed by a CCD camera too. The CCD camera with 640 x 480 pixels (pixel dimensions of 9.9 x 9.9 μm) and high sensitivity over a wide visible range was used in order to acquire the visible combustion. A BG-39 filter was placed in front of the CCD in order to shield it from the IR stimulation. This gave a detection window from approximately 400-700 nm. Synchronization between the engine and the ICCD and CCD was controlled by the delay unit linked with the signal coming from the angle shaft encoder. In order to have the absolute intensity of the flame emission spectra, these last were calibrated with Tungsten lamp with a known blackbody temperature, and the DeVos data were used [6, 7]. By the knowledge of the absolute flame emission intensity and the in-cylinder flame temperature, the soot emissivity was determined applying the Wien law: